1. Field of the Invention
The present invention relates to an electroconductive paste which is especially adapted to be printed onto a glass substrate and fired to form an anti-clouding heating wire or the like for a motorcar window.
2. Description of the Related Art
Anti-clouding heating wires for motorcar windows have from the standpoint of weather-resistance and price been heretofore formed by printing or coating electroconductive pastes containing silver, and sintering the pastes. However, the standard sizes of glass sheets of motorcar windows are different from each other, depending on the types of the motorcar. The resistance of the formed heating wires are required to have a constant value irrespective of the sizes of the glass sheets. Therefore, the specific resistances of the heating wires are required to be adjusted.
The specific resistance of heating wires which are applied to large-size motorcars and comply with low resistance specifications are from 2 to 3 μΩ·cm. On the other hand, the specific resistance of heating wires which are applied to small-size motorcars and comply with high resistance specifications are from 10 to 20 μΩ·cm. That is, the specific resistances range widely. The specific resistance of such heating wires are adjusted by increasing the amounts of glass frit in conductive pastes, adding a resistance-controlling agent, or the like. The resistance-controlling agent contains a metal, an oxide or the like which cannot form a solid-solution with the silver. They suppresse the sintering of silver contained in the conductive pastes, and thereby, the specific resistance is adjusted. The resistance-controlling agent is used especially for the heating wires which are formed according to high resistance specifications.
If the amount of glass frit is excessively increased or if the amount of the resistance-controlling agent which is added to adjust the specific resistance of a heating wire is excessively large, the glass frit or the resistance controlling agent is segregated on the surface (firing surface) of the formed heating wire. The glass frit or the resistance-controlling agent segregated on the surface of the heating wire remarkably deteriorates the solder-wetability of the heating wire. Thus, problems are caused in that the bonding strength between the heating wire and a lead wire or the like connected to the heating wire is reduced.